Field of the Invention
This invention relates to a focus detecting apparatus, and in particular to a focus detecting apparatus suitable for a photographic camera, a video camera or the like in which distance measurement can be accomplished at any position of an object.
Focus detecting apparatuses of various types have heretofore been proposed, and in principle, in any of them, distance measurement has been effected at the center of the picture plane of the finder field, i.e., on the optic axis of the objective lens, irrespective of the size of the distance measuring area. Therefore, there has been no problem when the main portion of an object to which the objective lens is to be focused is set at the center of the picture plane. However, when it is desired to set the main portion of the object at a position other than the center of the picture plane, it has been necessary to turn the camera so that the main portion of the object lies at the center of the picture plane and focus the objective lens, and then return the object to a desired position on the picture plane and re-set the picture plane. So, if a focus detecting apparatus which enables the distance measuring field to be moved is constructed, the above-noted problem will be solved, but in such case, the range in which distance measurement is possible, i.e., the length of the distance measuring field, must naturally be made great.
For example, when a light flux passed through the photo-taking lens is utilized and a light-receiving element for focus detection is fixed and focus detection is to be effected within this length of the distance measuring field, it is necessary to form the object image on the light-receiving element even if the imaged point of the object image moves. This has led to the problem that the light-receiving surface of the light-receiving element must have a wide area.
On the other hand, U.S. Pat. No. 3,553,455 shows an apparatus in which two images of an object (which have a parallax) are scanned by a slit. In addition, an apparatus in which two images formed by a light flux passed through an objective lens are scanned by a slit is known from Japanese Patent Publication No. 37957/1971. However, any of these apparatuses is such that the center of the distance measuring field coincides with the optic axis of the objective lens. That is, distance measurement is effected in the central region of the picture plane of the viewfinder.
Also, German Patent No. 2156617 discloses a technique in which the image is scanned, but distance measurement is effected at the center of the picture plane. As other focus detecting apparatuses, there are known U.S. Pat. Nos. 4,636,627, 4,634,255, 4,634,557, 4,670,645, 4,632,532 and 4,560,863.